In a general method of assembling components on printed circuit boards, the component leads are mounted through holes punched in the board for that purpose and the leads are then clinched, or bent, to hold the components in place during subsequent fluxing, lead cutting and soldering operations. The leads are soldered to the boards and the excess leads cut off. In many cases, the cut lead ends must then be re-soldered to cover them. These operations require hand labor and many steps, which of course is expensive. Also, solder is lost on the discarded lead ends. Further, when a double soldering step is required, the danger of board warpage, due to the high temperatures involved in soldering, is increased.
With the increased complexity of electronic devices, more components are mounted on the boards, which creates more problems. The clinched leads on the boards interfere with each other, necessitating a stepwise component mounting whereby part of the components are assembled, soldered and their leads cut off before mounting additional components.
Further, it is more difficult to remove faulty components which must be replaced or repaired when the leads have been clinched, without damaging the board or other adjacent components. Still further, access to both sides of the board is required.
Thus other ways of assembling printed circuit board components without clinching leads have been sought. U.S. Pat. Nos. 2,973,499 and 3,230,612 describe adaptors or griplets which are fitted into the board holes and which receive the lead ends and pinch or grip them to hold them in place. However, these methods require additional steps and parts which also add to the expense of manufacture.
U.S. Pat. No. 3,540,718 describes a component clamp and assembly jig whereby a sliced compressible pad, as of plastic foam, is pressed against the components to keep them in place during the soldering and lead cutting steps. This method is not entirely satisfactory either because it is difficult to maintain a uniform pressure against all components which may differ greatly in size and weight. Also, the pad tends to become impregnated with flux and solder after several operations and requires extensive cleaning to retain its resiliency.
Other means of holding the components down by means of pressure have also been tried. When all of the components are of near uniform size and weight, a bag loaded with small particles, e.g. beans or plastic shot, placed over the components will help keep them in place. However, this method also is unsatisfactory when both large and small components must be assembled in close proximity to each other, because the smaller components will not be held down properly and they may become loosened or even dislodged during soldering.
According to another method that has been tried, a molten wax coating is applied to the bottom of the board after the component leads have been inserted into the holes. After the wax cools and hardens, it holds the components in place while the leads are cut and soldered. This method has several advantages; it is particularly adaptable to automatic soldering operations, and the leads can be cut prior to soldering, so that solder covers all the lead ends in one operation. In addition, the wax acts as a flux for the soldering operation. The problems with the existing wax material are that the wax must be removed either with organic solvents, which poses storage and disposal problems, or with hot water, which requires a temperature of over 150.degree. F. which is expensive. In some instances also the soldering efficiency of the wax is inadequate for automated soldering operations, necessitating the application of a second flux over the wax coating.
Thus it would be desirable to provide an improved method of assembling components that will maintain unclinched components in place during soldering operations.